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Author Topic: marshall G30R CD clean channel volume problem  (Read 1005 times)

g1

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Re: marshall G30R CD clean channel volume problem
« Reply #15 on: February 09, 2020, 02:40:52 PM »
Ok, looking closer at the schematic, the reverb thing doesn't matter.  The reverb gets it's input signal after the switching IC, so it shouldn't be able to bypass the clean vol.

But I'd still like to establish that this is the correct schematic.  You do get reverb on the clean channel, don't you?
Here's a pic as yours didn't show.  The black line above the OD ch. controls show which are for OD channel.  Reverb is not included, but is over to the right, as it is 'global' and supposed to work for both channels.  Is this pic like your unit?



If the schematic is correct and the volume pot is not the source of the bleed, the only other place I can see would be the 2120 switching IC.

« Last Edit: February 09, 2020, 02:42:19 PM by g1 »

lordraptor1

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Re: marshall G30R CD clean channel volume problem
« Reply #16 on: February 09, 2020, 11:57:53 PM »
yes that is what the G30R CD looks like, sad i cant get pics to post even when it was resized to 253 KB.  yes the reverb works on both clean and distortion channels. i could have told you the reverb had nothing to do with it as i had unplugged the spring "tank" from the board and it had 0 effect on the clean channel volume neither did multiple brand new pots or a wire from pots ground lug direct to chassis.

lordraptor1

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Re: marshall G30R CD clean channel volume problem
« Reply #17 on: February 11, 2020, 10:03:46 AM »
Ok, looking closer at the schematic, the reverb thing doesn't matter.  The reverb gets it's input signal after the switching IC, so it shouldn't be able to bypass the clean vol.

But I'd still like to establish that this is the correct schematic.  You do get reverb on the clean channel, don't you?
Here's a pic as yours didn't show.  The black line above the OD ch. controls show which are for OD channel.  Reverb is not included, but is over to the right, as it is 'global' and supposed to work for both channels.  Is this pic like your unit?



If the schematic is correct and the volume pot is not the source of the bleed, the only other place I can see would be the 2120 switching IC.

thanks for the 2120 ic, i have also been messaging marshall about the issue to which i just now get a reply back from marshall:

Joanna Green <joanna.green@marshall.com>
To:
lordraptor1@yahoo.com

Feb 11 at 2:03 AM

Hello Fabian,

 

Regarding your question, it could be IC3 at fault as this is switching between the two channels.

 

Kind Regards

 

This email (including any attachments) may contain confidential information,
​privileged information, or constitute non-public information. Any use of this
​information by anyone other than the intended recipient is prohibited.
​If you have received this email in error, please inform the sender
​immediately and delete this information from your systems.

Marshall Amplification plc, registered in England, Registered number: 805676.
​Denbigh Road, Bletchley, Milton Keynes, MK1 1DQ, England.

JOANNA GREEN​
RETURNS/CUSTOMER SERVICES MANAGER
+44 (0)1908 375 411
DENBIGH ROAD, BLETCHLEY, ENGLAND, MK1 1DQ

 
This email (including any attachments) may contain confidential information,
​privileged information, or constitute non-public information. Any use of this
​information by anyone other than the intended recipient is prohibited.
​If you have received this email in error, please inform the sender
​immediately and delete this information from your systems.

Marshall Amplification plc, registered in England, Registered number: 805676.
​Denbigh Road, Bletchley, Milton Keynes, MK1 1DQ, England.

they to seem to think that IC3 ( this is the 2120 im assuming you were talking about in the schematic) COULD be at fault causing the issue. 


lordraptor1

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Re: marshall G30R CD clean channel volume problem
« Reply #18 on: February 11, 2020, 06:09:42 PM »
well good news bad news, good news i ordered the 2120D op amp i needed, bad news they are manufacturer discontinued and mouser and digikey were out of stock and not ordering any more of them bummer, all other outlets have a MOQ of at least 50 pieces o.O.  and to be honest im not even sure this will fix the issue with the amp but i have tried everything else i could think of. 

so i at least got the opamp i need on order before there were none left in the USA, i mean sure there are a lot of ones listed from chinese sellers but personal experience and word of mouth suggest a lot of those "china" sellers are selling fake op amp chips ( guess china has no shame in that they will even fake ICs.

g1

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Re: marshall G30R CD clean channel volume problem
« Reply #19 on: February 11, 2020, 08:33:44 PM »
Yes, I meant IC3.  NJM2121D is a higher spec'd chip that will work fine there.  Mouser has stock.

Agree that it will not necessarily fix the issue.  Two other IC's that are used for both channels (so could possibly cause the crosstalk Enzo mentioned) would be IC1, and IC5, both are type 4558D.

Jazz P Bass

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Re: marshall G30R CD clean channel volume problem
« Reply #20 on: February 12, 2020, 02:00:15 AM »
I would recommend 'grounding' the clean channel wiper.
That should kill the signal.
As yet, I have seen no resistance measurements of the circuit.
I would look at the junction of C49 & C 22 to ground.
What are the readings when the clean volume control is swept from min to max?

lordraptor1

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Re: marshall G30R CD clean channel volume problem
« Reply #21 on: February 12, 2020, 02:43:54 AM »
I would recommend 'grounding' the clean channel wiper.
That should kill the signal.
As yet, I have seen no resistance measurements of the circuit.
I would look at the junction of C49 & C 22 to ground.
What are the readings when the clean volume control is swept from min to max?

been there done that, grounding clean wiper had no change, C49 and C22  were tested and fond good but having had parts test good and be bad in the past i tried replacing C49 and C22 which again had no effect so i reinstalled the original parts.  only the op amps remain, and after talkign with marshall which only took them a week to respond to e-mail ( yeas that is sarcasm if you dont get it), they state it COULD be the IC3 2120D.  for those interested in this tidbit find and buy 2120S ICs while you can as they are manufacturer discontinued.

lordraptor1

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Re: marshall G30R CD clean channel volume problem
« Reply #22 on: February 12, 2020, 02:46:12 AM »
Yes, I meant IC3.  NJM2121D is a higher spec'd chip that will work fine there.  Mouser has stock.

Agree that it will not necessarily fix the issue.  Two other IC's that are used for both channels (so could possibly cause the crosstalk Enzo mentioned) would be IC1, and IC5, both are type 4558D.

already found a seller in the US that actually had NJM2120D so ordered some 4558 woulld have to wait till more $$$ presents itself.  i will look into the 2121D

lordraptor1

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Re: marshall G30R CD clean channel volume problem
« Reply #23 on: February 12, 2020, 02:49:58 AM »
Yes, I meant IC3.  NJM2121D is a higher spec'd chip that will work fine there.  Mouser has stock.

Agree that it will not necessarily fix the issue.  Two other IC's that are used for both channels (so could possibly cause the crosstalk Enzo mentioned) would be IC1, and IC5, both are type 4558D.
just looked at the 2120D on mouser but like the original 2120D it has also been manufacturer discontinued.

makes me wary of marshall  tube amps from teh JCM900 to tsl line which all used the 2120D's in them.  i mean really manufacturer discontinuing parts and i havent seen anything from marshall in the way of a service bulletin regarding replacements for the ics should they fail.

phatt

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Re: marshall G30R CD clean channel volume problem
« Reply #24 on: February 12, 2020, 07:00:13 AM »
I would recommend 'grounding' the clean channel wiper.
That should kill the signal.
As yet, I have seen no resistance measurements of the circuit.
I would look at the junction of C49 & C 22 to ground.
What are the readings when the clean volume control is swept from min to max?

Bingo  I Agree   :tu:
What you think might be ground may not actually be at ground.
That is why we use meters. 8|

I'd add to that,, and suggest measure the DC resistance from volume wiper to the track ground on PCB and then also to the main filter caps common/ground point,, and then again to Chassis/Case.
They should ALL be within a bees dick of of Zero Ohms.

THEN and only then will you know it's not the volume control.

Frankly I'd suggest save yourself the pain and $$$,,just leave it and pull the plug half out as that will ground the input and kill all sound. How hard is that?
Phil.


lordraptor1

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Re: marshall G30R CD clean channel volume problem
« Reply #25 on: February 13, 2020, 08:37:35 PM »
I would recommend 'grounding' the clean channel wiper.
That should kill the signal.
As yet, I have seen no resistance measurements of the circuit.
I would look at the junction of C49 & C 22 to ground.
What are the readings when the clean volume control is swept from min to max?

Bingo  I Agree   :tu:
What you think might be ground may not actually be at ground.
That is why we use meters. 8|

I'd add to that,, and suggest measure the DC resistance from volume wiper to the track ground on PCB and then also to the main filter caps common/ground point,, and then again to Chassis/Case.
They should ALL be within a bees dick of of Zero Ohms.

THEN and only then will you know it's not the volume control.

Frankly I'd suggest save yourself the pain and $$$,,just leave it and pull the plug half out as that will ground the input and kill all sound. How hard is that?
Phil.

just to clarify i know exactly what and how to test this isnt my first rodeo.  and i will say this again i have tested EVERYTHING and i DO know which leg of the pot is the ground ( not hard to see if you know how to look at the traces on the PCB and see which leg leads to ground plane.  i did test pot and the wiper itself does 0 as does the other 5 brand new 1M pots i have.

fact is i have trouble shot this thing for over a week prior to even making a post here on the forum.  at the end of the day the only thing i have not checked or replaced that according to the schematic COULD be at fault are the ic's and based on the condition of the input jack ( and also the footswitch jack next to and share some traces) i have no difficulty believing that the faulty jacks may have cause an ic to fail.

i already have the 2120D ic coming,and i have a friend thaat says he might have one 4558 left in his stash he said i could have (yay for free ).  i normally dont mess with ss amps because of the headaches but i think after this im not only go back to my guns of refusing ss amp repairs but also refuse modern tube amps that have IC's in them. 

so much easier repairing, troubleshooting and building from scratch vintage, cloned vintage tube amps.

Jazz P Bass

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Re: marshall G30R CD clean channel volume problem
« Reply #26 on: February 14, 2020, 01:13:51 AM »
We;; that was a big help.

lordraptor1

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Re: marshall G30R CD clean channel volume problem
« Reply #27 on: February 15, 2020, 04:09:02 PM »
We;; that was a big help.

what is not a big help is people not reading and understanding that i have already checked the ground and tried multiple brand new pots so ground was ruled out already, nor have they read or understood that all the other pots seem to work fine.  marshall finally got back and said it COULD be the 2120 ic switching op amp at fault so im going to change it.  other than that the only thing i could do would be start shotgunning ic's and electrolytics of course at that point it just isnt worth it when most the amp COULD sell for being that it is solid state and not tube is $200.  i just thought some ss guru would have some xp with this amp and would have some insights as to causes but seems like most point to ground followed by the pot itself at fault ( again both ruled out already).  i mean at this point im so frustrated and the check the ground, check the pot, and the may not actually be ground comments are not helping and are in no way useful and are in fact COUNTER productive.

phatt

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Re: marshall G30R CD clean channel volume problem
« Reply #28 on: February 15, 2020, 11:55:26 PM »
Sorry but it's the wrong forum for anger management issues.

We try to help sometimes we get it wrong,, but we don't have the amp in front of us. xP

If you are well versed in Valve circuits then you would know how important ground return paths are.
Yes every Common point may well be at Ground/Zero **BUT it's the path each section takes back to Main Common point that can make or break a design.**

Tiss why I suggest you take a wire from Volume Common back to to central PSU (where the 2 main filter caps join) and see if it resolves the issue. As you have a new chip coming I hope it works out for you if not then I'd go hunting Ground Paths.

I've worked on enough gear to know that many problems are ground related.
Just as wire layout is important in Valve gear it's no different with SS PCB layout. Unlike lead layout in p-p valve circuits which can be easy rewired,, PCB is Fixed once printed and resolving ground return path issues becomes a lot harder.

I fixed a mates Pro junior Valve amp,, The most sensitive audio trace ran right along side the screen supply trace. :duh
Hence the damn thing hummed like crazy. :trouble
I has to cut a few tracks and hand wire to shut the thing up.
So that was a Fender design layout stuff up.
Without a deep understanding of ground plane layout even the big name gear is often full of suspect design issues.
I wish you well and hope you resolve the issue.
Phil.

phatt

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Re: marshall G30R CD clean channel volume problem
« Reply #29 on: February 16, 2020, 01:42:52 AM »
Oh yeah,, just re reading to check what I may have missed.
You mentioned the input and switch sockets had some issues.
up shot is the control voltage that runs IC2&3 may not be working correctly which would likely cause the issue you have.
I'd be checking that before replacing chips. Control is around TR1.
hope it helps,
Phil.